By biochar, it is meant according to the invention, a stable carbon-rich solid derived from a heat treatment of a biomass suitable for numerous industrial applications. Thus, it constitutes, but is not limited to, a combustible with a high calorific value, representing a new alternative in the field of renewable energies. It also constitutes a fertilizer for agricultural use for soil amendment. It also constitutes a product intended for the chemical industry, for example as a catalyst. It is further an excellent adsorber and constitutes a purifier, a decolorant, a decontaminant and/or a deodorant, usable in numerous industrial fields. It may be shaped in any format depending on its destination, such as a powder, grains . . .
The method of the invention is described more specifically with reference to a lignocellulosic biomass, but by analogy, it may be applied to other biomasses.
Methods for transforming a lignocellulosic biomass into combustibles are already known involving in particular a torrefaction step. The torrefaction of a biomass consists in heating it gradually to a moderate temperature, generally between 190° C. and 250° C., in an oxygen-free atmosphere, and possibly under pressure. This treatment results in an almost complete elimination of water from the biomass and in a partial modification of its molecular structure, causing a change of some of its properties. In particular, this heat treatment produces a depolymerization of hemicellulose, making the torrefied biomass almost hydrophobic and friable, while improving its calorific value.
Thus, the document EP2287278A2 describes a method for torrefying a lignocellulosic biomass comprising a step of drying the biomass so as to remove about 95% of moisture, then a torrefaction step in a reactor brought to a temperature of 100-1000° C. in theory, 220-300° C. in practice, at a pressure of 1-50 bar, preferably 5-20 bar, in an oxygen-free atmosphere, and finally a step of cooling the torrefied biomass, this method providing for a gas recycling system.
There is also known according to the document WO2013/003615A2, a device for torrefying a hemicellulose-rich biomass such as wood, and a method for treating this biomass implemented in this device, comprising a biomass drying step, a torrefaction step carried out at a temperature of 200-250° C., at a pressure of at least 3 bar, in an inert atmosphere, and a cooling step. The device is constituted by a vertical body in which is disposed a superposition of plates constituting treatment compartments of the biomass. These compartments are equipped with apertures for allowing the biomass, being treated or already treated, to flow and the treatment gases or products may be evacuated via pipes in order to be recycled.
According to the article J. Wannapeera and N. Worasuwannarak, Journal of Analytical and Applied Pyrolysis 96 (2012) 173-180, the authors have studied, on a laboratory scale, namely on a few grams, the effect of pressure in a method for transforming, by torrefaction, a biomass based on Leucaena leucocephala, a tropical tree. This method comprises the following steps:                The biomass is shredded then ground into particles of a size <75 μm;        The particles are dried afterwards in a vacuum oven at 70° C. for 24 h;        The particles are placed in a reactor under inert atmosphere, which is then introduced and maintained in a furnace at a temperature of 200-250° C. and a pressure of 1-40 bar, for 30 min;        After these 30 min, the reactor is immersed in water in order to stop the reaction;        The product derived from this carbonization is dried in an oven for 2-3 h then analyzed.        
The highest values of the higher calorific value (HCV) are obtained for a solid derived from a torrefaction at a temperature in the order of 250° C. and a pressure of 40 bar. These works have highlighted the favorable effect of pressure on torrefaction reactions in these conditions.
Known torrefaction treatments under pressure such as those described before produce solids having a high lower calorific value (LCV), generally in the order of 19 to 23 MJ/kg. The LCV of a solid obtained according to the method described in EP2287278A2 is actually in this order. The authors state herein that their torrefaction method would result in a mass reduction of 30% with a loss of 10% of the overall energy, which means that the energy of the obtained solid, corresponding to 90% of the energy of the initial dried biomass, is concentrated in 70% of the mass of the initial dried biomass, which leads to a concentration of the LCV per unit mass of 0.9/0.7, namely 1.28. The reported biomass drying ratio being 5%, equivalent to that of a commercial wood granulate whose LCV is in the order of 15 to 18 MJ/kg, the LCV of the obtained solid according to EP2287278A2 is in the order of 19 to 23 MJ/kg. Besides, these values are those reported by numerous developers in this field.
However, there is still a growing need to develop methods which are more effective and less energy-consuming, with investments which are less expensive, easier to control and which allow to obtain a combustible with a better quality.
The authors of the present invention have discovered that the implementation of a torrefaction in specific conditions allows initiating a spontaneous exothermic phenomenon, producing a combustible solid whose LCV is very high, much higher than that of combustibles resulting from the transformation methods discussed before. Moreover, this combustible solid has a very high carbon content, generally higher than 80% by mass and a reduced oxygen content, in the order of 10% by mass or less. The authors have also observed that this phenomenon would occur with numerous types of biomass.
This exothermic phenomenon is prevented in known methods because it is considered as an unfavorable factor in the energy balance. The authors of the invention have actually demonstrated that it is the development of this phenomenon which allows producing a biochar which is more carbon-rich, and therefore more calorific.
Two conditions are essential so that this phenomenon occurs. They lie in an accurate control of the grain-size distribution of the involved biomass and in the drying of the latter before the torrefaction step, this drying having to be complete. Hence, the prior drying step has to retrieve all moisture from the biomass, in order to reach a moisture content close to 0, and always lower than 10% by mass. The more the treated biomass is close to the anhydrous condition, the more the method is effective.